See example at

http://weboom.wordpress.com/2009/07/01/what-is-block/

For example, assume that base.html has

{% block yourblock %}

Here, your block content in base.html

{% endblock %}

———-

In base_child.html can have their own version of yourblock content

{% extends “base.html” %}

{% block yourblock %}

Here, the base.html content can be overwritted with this.

{% endblock %}